Add --no-decode option.

This disables most decoding of the contents of Mode S messages, aircraft tracking, and some output modes that depend on them.
It's intended for edge receivers that just forward to a central hub rather than processing data locally.

dump1090.h

@@ -317,6 +317,7 @@ struct {                             // Internal state
     int   metric;                    // Use metric units
     int   mlat;                      // Use Beast ascii format for raw data output, i.e. @...; iso *...;
     int   interactive_rtl1090;       // flight table in interactive mode is formatted like RTL1090
+    int   no_decode;                 // Disable decoding and aircraft tracking
 
     // User details
     double fUserLat;                // Users receiver/antenna lat/lon needed for initial surface location